Low pressure olefin recovery process

ABSTRACT

A low-pressure olefins recovery process and plant are described. The feed gas is compressed and distilled at a primary distillation pressure. The overhead stream is chilled at a pressure less than 30 kg/cm2 (430 psia) to partially condense the overheads. The primary distillation tower is refluxed with at least a portion of the condensate. The overhead vapor is further chilled and partially condensed and the condensate is fed to a demethanizer. The remaining vapor is cooled in a cold section and the resultant liquid is phase-separated and expanded to provide refrigeration for the cold section. The expanded vapor from the cold section is recycled to the process gas compressor. The bottoms streams from the primary distillation zone and the demethanizer are fractionated into respective streams consisting essentially of ethylene, ethane, propylene, propane, C4&#39;s, and C5+.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/884,659, filed on Jul. 2, 2004, which is incorporated byreference herein.

FIELD

The present embodiments relate to an improved olefin recovery process,and more particularly to an olefin recovery process employing a lowpressure front end distillation with a low pressure chilling train, alow pressure deethanizer and a low pressure demethanizer, to minimizethe number of pieces of equipment that are needed to recover the olefinsand to reduce the capital cost of the equipment.

BACKGROUND

Olefins are produced in a feed gas that originates from catalyticreactors and/or thermal cracking furnaces that are well known in theart, such as, for example, the SUPERFLEX™ process of Kellogg Brown &Root LLC, the catalytic process for cracking methanol, the deepcatalytic cracking process, FCC reactors, and the like.

The olefin containing feed gas must be processed to separate and recoverthe olefins from various amounts of other gases, which can includehydrogen, methane, ethane, propane, butanes, and pentanes-and-heavierhydrocarbons. The feed gas can also include contaminants such as carbondioxide, acetylene, arsenic, mercury, carbonyl sulfide, nitrogen oxides,oxygen, and the like, which must generally be removed or treated.

In some conventional olefin recovery processes, the feed gas iscompressed and fractionated in a front-end, heat-pumped deethanizer ordepropanizer, employing relatively high pressures on the order of400-500 psia in the case of the front end deethanizer and 400-600 psiain the case of the front end depropanizer. Schematic diagrams showingthese prior art olefin recovery schemes are illustrated in FIGS. 1 and2. Front-end demethanizer processes have also been used, employingpressures of 500-600 psia. High pressures are required in these olefinrecovery schemes to obtain high ethylene/propylene recoveries. Theserelatively high pressures typically require four compressor stages, andan expander-recompressor is employed around the cold sectionrefrigeration system. The high pressure of the equipment and the numberof pieces of equipment increases the capital cost of the equipment. Itwould be desirable to reduce the number of pieces of equipment, as wellas the cost.

In the processing of feed gases containing trace amounts of nitrogenoxides, such as, for example, in FCC effluent, there is also a potentialsafety hazard that must also be considered. A reactor is normally usedto remove nitrogen oxides before the process gas is sent to the coldsection, but leakage or upset or other malfunction can result innitrogen oxides being present in the cold section. The presence ofnitrogen oxide at temperatures below about 105° C. can result in theformation and accumulation of nitrated gums in the coldest cold boxexchanger. Nitrated gums are unstable and can explode if thermally ormechanically shocked. Temperatures below 105° C. in the cold box shouldbe avoided to minimize the possibility of nitrated gum formation.

One goal is to maximize ethylene/propylene recovery while at the sametime minimizing energy consumption and other operating costs. Often, thelower temperatures required to reduce the loss of olefin in tail gasand/or hydrogen product streams will require additional power, creatinga trade-off between power consumption and olefin losses. To maximizeheat and refrigeration recovery, a relatively large number of heatexchangers may be employed. Furthermore, higher olefin recovery ratescan necessitate the use of colder temperatures below the temperature atwhich nitrated gums can form in the cold box.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will be better understood in conjunction withthe accompanying drawings as follows:

FIG. 1 (prior art) is a schematic flow diagram of a conventionalhigh-pressure olefins recovery process with a front-end depropanizer.

FIG. 2 (prior art) is a schematic flow diagram of a conventionalhigh-pressure olefins recovery process with a front-end deethanizer.

FIG. 3 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer.

FIG. 4 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddeethanizer.

FIG. 5 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer/deethanizer in series.

FIG. 6 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer and a depropanizer reflux pump.

FIG. 7 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer and an ethylene absorber.

FIGS. 8A and 8B, taken together and referred to herein collectively asFIG. 8, are example simulation diagrams of the low pressure olefinsrecovery process of FIG. 3 showing pressure (oval/circular balloons) andtemperature (hexagonal balloons) of selected streams, as discussed inExample 1 below.

FIGS. 9A and 9B, taken together and referred to herein collectively asFIG. 9, are example simulation diagrams of the low pressure olefinsrecovery process of FIG. 7 showing pressure (oval/circular balloons) andtemperature (hexagonal balloons) of selected streams, as discussed inExample 2 below.

The present embodiments are detailed below with reference to the listedFigures.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Before explaining the present embodiments in detail, it is to beunderstood that the embodiments are not limited to the particularembodiments and that they can be practiced or carried out in variousways.

The present embodiments are olefin recovery processes that heat pumpsthe front-end distillation tower at a relatively low pressure, with goodethylene recovery and energy consumption. The process schemes result infewer pieces of equipment, lower pressure ratings and lower capitalcosts.

In one aspect, an embodiment provides a process for recovering olefinsfrom a feed stream. The process includes:

-   -   supplying the feed stream at a primary distillation pressure,        including, if required, compressing the feed stream in at least        one primary compression stage;    -   distilling the feed stream at the primary distillation pressure        in a primary distillation zone to obtain a primary overhead        vapor stream enriched in ethylene and one or more ethylene-lean        bottoms streams;    -   chilling the primary overhead vapor stream at a pressure less        than 30 kg/cm² (430 psia), preferably less than 28 kg/cm² (400        psia), in a first cooling stage to recover a first partial        condensate stream and a first-stage vapor effluent;    -   refluxing the primary distillation zone with at least a portion        of the first partial condensate stream;    -   further chilling the first-stage vapor effluent to recover at        least a second partial condensate stream and a second-stage        vapor effluent;    -   feeding the second partial condensate stream and any remaining        portion of the first partial condensate stream to a demethanizer        to recover a methane-rich overhead stream and a bottoms stream        essentially free of methane and lighter components;    -   fractionating the bottoms streams from the primary distillation        zone and the demethanizer into respective streams consisting        essentially of hydrocarbons selected from the group consisting        of ethylene, ethane, propylene, propane, C₄'s, C₅₊ and        combinations thereof, and further chilling the second-stage        vapor effluent in a cold section and phase-separating the        resulting mixed vapor-liquid stream in one or more stages to        obtain additional condensate and a vapor tail gas stream        essentially free of ethylene, wherein the additional condensate        is vaporized at a relatively lower pressure to provide        refrigeration for chilling and to form a low pressure recycle        vapor stream.

The process can include passing the compressed feed stream and/or theprimary overhead vapor stream in contact with a catalyst to removecontaminants such as acetylene, arsenic, mercury, carbonyl sulfide,nitrogen oxides, oxygen, combinations thereof, and the like.

The primary overhead vapor stream can be compressed in a secondarycompression stage to a discharge pressure effective to provide refluxfor the primary distillation zone. The primary distillation pressure canbe from 7 to 21 kg/cm² (100 to 300 psia) and the discharge pressure fromthe secondary compression stage is preferably greater than the primarydistillation pressure and less than 28 kg/cm² (400 psia). The dischargepressure from the secondary compression stage can be from 3.5 to 7kg/cm² (50 to 100 psia) greater than the primary distillation pressure.One combination includes a primary distillation pressure from 7 to 11kg/cm² (100 to 160 psia) and a secondary compression stage dischargepressure from 10.5 to 17.5 kg/cm² (150 to 250 psia).

The overhead stream from the demethanizer is preferably recycled intothe primary overhead vapor stream upstream of the secondary compressionstage. The demethanizer can consists essentially of an unrefluxedstripper column.

The low pressure recycle vapor stream from the cold section can beadvantageously recycled into the feed stream upstream of at least onestage of the primary compression stage or stages. The process caninclude contacting a stream, selected from the mixed vapor-liquidstream, the vapor tail gas stream and a combination thereof, with aheavier hydrocarbon stream lean in ethylene to absorb residual ethylenefrom the stream into the heavier hydrocarbon which is recycled in thelow pressure recycle vapor stream. The heavier hydrocarbon stream canconsists essentially of liquid ethane, propane, or a combinationthereof.

In one embodiment, the primary distillation zone comprises adepropanizer. In this embodiment, the process can include fractionatingthe bottoms stream from the depropanizer in a debutanizer to obtainrespective streams consisting essentially of C₄'s and C₅'s and heavierhydrocarbons, and fractionating the bottoms stream from the demethanizerin a deethanizer, a C₂ splitter and a C₃ splitter to obtain respectivestreams consisting essentially of ethylene, ethane, propylene andpropane. The deethanizer is preferably refluxed with a side draw fromthe C₂ splitter

In an alternate embodiment, the primary distillation zone comprises adeethanizer and the process includes fractionating the bottoms streamfrom the deethanizer in a depropanizer, a C₃ splitter and a debutanizerto obtain respective streams consisting essentially of propylene,propane, C₄'s and C₅'s and heavier hydrocarbons, and fractionating thebottoms stream from the demethanizer in a C₂ splitter to obtainrespective streams consisting essentially of ethylene and ethane.

In a further embodiment, the primary distillation zone comprises adepropanizer and a demethanizer, and the process includes fractionatinga bottoms stream from the depropanizer in a debutanizer to obtainrespective streams consisting essentially of C₄'s and C₅'s and heavierhydrocarbons, fractionating a bottoms stream from the deethanizer in aC₃ splitter to obtain respective streams consisting essentially ofpropylene and propane, and fractionating a bottoms stream from thedemethanizer in a C₂ splitter to obtain respective streams consistingessentially of ethylene and ethane. In this embodiment, the process caninclude partially condensing overhead vapor from the depropanizer toform C₄-lean vapor and liquid streams, feeding the C₄-lean vapor streamto the deethanizer, and refluxing the depropanizer with the C₄-leanliquid stream.

With reference to the figures, FIG. 3 is an example schematic flowdiagram of a low-pressure olefins recovery process according to apresent embodiment with a front-end depropanizer. In the front enddepropanizer embodiment of FIG. 3, the embodiments supply anolefin-containing feed gas stream 300 to the suction of the first stageprocess gas compressor (PGC) 302 which operates in series with a secondstage PGC 304 to produce an intermediate feed stream 306 at a pressureof 7 to 10.5 kg/cm² (100 to 250 psia), preferably 9.1 kg/cm² (130 psia).The feed stream 300 is typically washed in water and oil (not shown) toremove soot and heavy oil in a conventional manner, as well as to coolthe stream 300 to ambient temperature or below, as required.Conventional interstage cooling (not shown) and liquid removal (notshown) can also be employed, if desired.

The intermediate feed stream 306 is optionally treated in conventionalreactor unit 308, which can include an amine and/or caustic wash toremove acid gases and catalyst and/or adsorbent bed(s), such asimpregnated carbon, nickel sulfide or the like, to remove arsenic,mercury, carbonyl sulfide, nitrogen oxides, oxygen or othercontaminants. The unit 308 can include a conventional catalyst to reactacetylene and a portion of methyl acetylene and propadiene (MAPD), anddesiccants such as molecular sieve, alumina or the like, or a glycolsystem, to remove water. The gas can be further cooled before feeding tothe front-end distillation tower 310.

The tower 310 can be a heat-pumped depropanizer, as exampled in theembodiment of FIG. 3, used to remove C₄'s and heavier hydrocarbons fromthe rest of the feed gas. The tower 310 is generally operated without anoverhead condenser, using process condensate for reflux. The tower 310overhead vapors in line 312 are compressed in the heat pump compressorstage 314 to a pressure of 10.5 to 24.5 kg/cm² (150 to 350 psia),preferably about 14 kg/cm² (200 psia), or as required by other processrequirements, such as hydrogen delivery pressure, for example. Thecompressed overhead vapors can then be reacted over a conventionalcatalyst system 316 to remove acetylene and a portion of MAPD, if notremoved in unit 308.

The compressed gas is successively cooled in chilling train 318 to theappropriate temperature, e.g. −18° C. (0° F.) in the case of thefront-end depropanizer example, and the condensed liquid is phaseseparated. A portion of this liquid is returned via line 320 to refluxthe tower 310. The remaining portion of the liquid is supplied via line322 to demethanizer 324. The remaining vapor is further cooled asrequired, preferably to about −71° C. (−95° F.), and the condensedliquid is phase separated and supplied in line 322 to the demethanizer324 together with the previously mentioned liquid from the earlier phaseseparation following the initial partial condensation. The remaininggases are supplied via line 326 to cold box 328 where they are furthercooled to a temperature of −95° to −130° C. (−140° to −200° F.),preferably −115° C. (−175° F.), and phase separated in drum 330 and/oradditional stages (not shown). The condensed liquid is expanded acrossJoule-Thompson valve 331 and vaporized at low pressure in the cold box328 to provide the refrigeration required in the condensation step.After vaporization, the gas, which contains appreciable ethylene, isrecycled via line 332 to the process gas compressor 302 to minimizeethylene losses. The vapor from the drum 330 is expanded acrossJoule-Thompson valve 334, passed through cold box 328 for recovery ofrefrigeration, and produced as an ethylene-lean tail gas 336 rich inmethane and hydrogen.

The demethanizer 324 can be a low pressure stripping tower with fewtrays. The low pressure stripping tower with few trays produces abottoms 338 that are essentially free of methane and lighter components.The overhead vapors 340 can be recycled, after reheating if required, tothe suction of the heat pump stage 314 via line 312. Alternatively, thedemethanizer 324 can be a refluxed tower (not shown) and the overheadmethane-rich stream 340 can be further cooled for additional ethylenerecovery and/or optionally expanded and used for fuel gas.

The bottoms stream 342 from the tower 310 can be supplied to aconventional debutanizer 344 that produces an overhead C₄ product 346and a bottoms gasoline or C₅₊ product stream 348.

The bottoms stream 338 from the demethanizer 324 is sent to deethanizer350. The deethanizer 350 is preferably operated at a relatively lowpressure, such as, for example, 4.2 to 7.7 kg/cm² (60 to 110 psia), forexample about 5.0 kg/cm² (72 psia) at the top, and refluxed from theethylene-ethane splitter (C₂ splitter) 352. In this configuration, thedeethanizer 350 does not require a condenser that is conventional inother designs. The overhead vapor stream 354 is supplied to the C₂splitter 352, which is operated to produce a high quality ethyleneproduct stream 356 overhead and a bottoms stream 358 of essentially pureethane. If desired, the deethanizer 350 and C₂ splitter 352 can be amechanically integrated column as described in U.S. Pat. No. 6,077,985to Stork, which is hereby incorporated herein by reference. Thedeethanizer bottoms stream 360, which can if desired include an MAPDreactor system (not shown), goes to a conventional C₃ splitter 362 forproducing overhead propylene stream 364 and bottoms propane stream 366as required.

The C₂ splitter 352 in this example can be heat pumped and coupled withethylene refrigeration compressor 368, which can be a two-stage unitused to provide −73° F. (−100° F.) refrigeration used elsewhere in theprocess. Efficient use is made of the refrigeration available fromvarious process streams and reboiler duties using conventionaloptimization schemes well known in the art to reduce the overall energyconsumption in the process.

The C₃ splitter 362 can be either a low pressure, heat pumped tower or astandard cooling water-condensed tower as determined by economics. Thepropylene is used to provide refrigeration at about −40° C. (−40° F.)used elsewhere in the process.

The ethylene recovery from the FIG. 3 illustration is in the range of98-99 percent, depending on the selected pressures and temperatures. Themain ethylene losses are in the tail gas 336 leaving the −95° to −130°C. (−140° to −200° F.) drum 330.

FIG. 4 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddeethanizer. FIG. 4 illustrates principles of the present embodiments inthe context of a front-end deethanizer. In one embodiment, the tower 410is a deethanizer and the bottoms stream 442, which is rich in propanesand heavier hydrocarbons, is supplied to depropanizer 470, which can bea single or dual tower system. The overheads stream 472 is supplied tothe C₃ splitter 462, while the bottoms stream 474 is supplied todebutanizer 444 as in the FIG. 3 embodiment. Since the demethanizer 424bottoms stream 438 is essentially free of propanes, it can be supplieddirectly to the C₂ splitter 452.

FIG. 5 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer/deethanizer in series. FIG. 5 illustrates the principles ofthe present embodiments in the context of a front-end depropanizer 510Aand deethanizer 510B operated in series. The depropanizer 510A can beoperated with a conventional overhead reflux condenser (not shown) andreflux drum 576. Overhead vapor stream 578 is supplied to deethanizer510B, which is heat pumped as in the FIG. 4 embodiment. The bottomsstream 542 from the depropanizer 510A is supplied to the debutanizer544, while the bottoms stream 560 from the deethanizer 510B is supplieddirectly to the C₃ splitter 562. If desired, in this embodiment, aportion of the overhead vapor 512 from the deethanizer 510B and/or thevapor from the first cooling stage in the chilling train 518 can beexported as a dilute ethylene product stream.

FIG. 6 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer and a depropanizer reflux pump. FIG. 6 further illustratesthe principles of the present embodiments in the context of a front-enddepropanizer wherein the process gas compression is limited to twostages. In one embodiment, the discharge pressure of the second stagePGC 604 is about 10.5 to 24.5 kg/cm² (150 to 350 psia), more preferablyabout 12.6-14 kg/cm² (180-200 psia), and the overhead vapor 612 from thedepropanizer tower 610 is supplied to the chilling train 618 essentiallyat the pressure of the tower 610, preferably about 10.5 to 11.2 kg/cm²(150 to 160 psia), especially about 10.85 kg/cm² (155 psia), withoutfurther compression. A reflux pump 680 is used to return the liquidstream 620 recovered from the chilling train 618 to the tower 610. Theoverhead vapor stream 640 from the demethanizer 624, which is not at ahigh enough pressure to be introduced into the depropanizer overheadstream 612, is instead heated in (or outside) the cold box 628 torecover refrigeration and recycled in line 632 to the suction of thefirst process gas compressor 602. This embodiment has the advantage ofeliminating the need for a third process gas compressor stage requiredfor heat pumping the tower 610 in the other embodiments.

FIG. 7 is an example schematic flow diagram of a low-pressure olefinsrecovery process according to a present embodiment with a front-enddepropanizer and an ethylene absorber. The embodiment of FIG. 7 uses afront-end depropanizer with an ethylene absorber 782 to further reduceethylene losses in tail gas stream 736. Ethylene absorber 782 ispreferably a simple tower without a condenser or reboiler used in placeof the separation drum 330 in FIG. 3. Ethane from line 758 (or from canbe supplied via line 784, cooled in exchanger unit 786 and introduced asa wash liquid to the top of the ethylene absorber 782, at about the samepressure and temperature as the absorber 782, e.g. 14 kg/cm² (200 psia)and −95° C. (−140° F.). The ethane-ethylene liquid from the absorber 782is collected in line 788 and recycled to the cold box 728, line 732 andprocess gas compressor 702. Alternatively, the ethane feedstock via line790, if sufficiently pure or after being appropriately purified, orpropane product via line 792, is used as the wash liquid. Using thisembodiment, ethylene recoveries of 99 percent and higher are achieved,preferably at least 99.8 percent, while at the same time avoiding theuse of extremely low temperatures. The ethylene absorber 782 is notlimited to use with the front-end depropanizer scheme of FIG. 7, and canbe utilized with any of the embodiments of FIGS. 3-6 in place of thecold box separator drum.

The embodiments achieve a reduction in the number of pieces of equipmentthat are used in the process, and thus concomitantly reduce the capitalcost. For example, only two or three stages of process gas compressionare used, compared to four or more in the conventional high-pressureprior art process. By operating a heat pumped, low-pressure initialdistillation tower, the condenser and reflux drums and pumps aregenerally eliminated, and the tower has a relatively low number oftrays. By coupling the deethanizer and C₂ splitter in the embodiment ofFIG. 3, a condenser and reflux drum are eliminated and the C₂ splitterreboiler duty is significantly reduced, while the C₂ splitter condenserduty increases only slightly. By using the ethylene absorber in the FIG.7 embodiment, the process can avoid nitrated gum formation temperatureswhile still maintaining high ethylene recovery and low powerconsumption. Very few heat exchangers are needed in the present process,yet it recovers refrigeration efficiently. If desired, the finalpressure profile can be adjusted to eliminate pumps for the depropanizerbottoms, the demethanizer bottoms and the deethanizer bottoms, and noreflux pumps are required for the depropanizer, deethanizer,demethanizer and C₂ splitter. Also, waste quench water heat can be usedfor reboiling the depropanizer, saving steam costs. Furthermore, byoperating at relatively low pressures, the present invention avoids theneed to use a coupled expander-recompressor (or expander-generator) torecover compression around the cold box as in prior art olefin recoveryschemes that operated at high pressure.

EXAMPLE 1

The embodiment of FIG. 3 was simulated on a commercial Aspen simulatorusing the simulation diagram seen in FIGS. 8A and 8B (“FIG. 8”) withselected pressures (oval balloons) and temperatures (hexagonal balloons)as indicated. The feed is in the form of gas and liquid streams havingthe compositions summarized in Table 1. TABLE 1 Vapor Feed Liquid FeedComponent (mol %) (mol %) H₂ 6.4 0.0 N₂ 0.4 0.0 CO₂ 0.1 0.0 H₂S 0.2 0.0CH₄ 5.8 0.0 C₂H₄ 13.3 0.3 C₂H₆ 2.5 0.1 C₃H₆ 20.1 1.8 C₃H₈ 6.4 0.71,3-Butadiene 0.1 0.0 1-Butene 10.0 3.0 i-Butane 9.2 2.2 n-Butane 2.91.0 C₅₊ 17.6 90.6 H₂O 5.0 0.3 Total Flow 8200 1000 (kmol/hr)

Three-stage process gas compression is used in a low-pressure recoverysystem. The front-end depropanizer 310 is operated at about 7 kg/cm2(100 psia) riding on the third stage PGC 314 suction. The third stagePGC 314 discharge pressure is about 14 kg/cm2 (200 psia). The acetylenereactor is disposed downstream from the third stage PGC 314 to converttotal acetylene to ethylene and ethane, and also to convert part of MAPDto propylene and propane. The acetylene reactor effluent is partiallycondensed against −20.8° C. (−5.4° F.) propylene refrigerant and part ofthe liquid provides reflux to the depropanizer 310. The process gas isfurther chilled against propylene and ethylene refrigerant to −71.7° C.(−97° F.). Condensed liquid is sent to the demethanizer 324 (sanscondenser). Non-condensed vapor is chilled down to 126° C. through coldbox exchanger 328. This partially condensed stream is sent to drum 330to separate the Joule-Thompson recycle liquid from the tail gas vapor.The tail gas from the drum 330 overhead, consisting essentially ofmethane and lighter components but also containing some ethylene, isused as fuel gas after recovery of refrigeration in the cold box 328exchangers. The Joule-Thompson recycle liquid from the bottom of thedrum 330, consisting essentially of ethylene and some methane, is sentback to the suction of the second stage PGC 304 after recoveringrefrigeration through the cold box 328 exchangers. The demethanizer 310is operated at about 7.7 kg/cm2 (110 psia), riding on the suction of thethird stage PGC 314 to recover ethylene. The bottoms from thedemethanizer 310 consist essentially of ethane, ethylene, propane andpropylene and are sent to deethanizer 350, C2 splitter 352 and C3splitter 362 to recover polymer grade ethylene and propylene. Ethanefrom the C2 splitter 352 bottoms can be combined with the tail gas 336and eventually sent to the fuel gas system.

One advantage of this system is that low pressures are used. The maximumPGC discharge pressure is 14 kg/cm2 (200 psia), and no −100° C. (148°F.) refrigeration level is required, yet good ethylene recovery isachieved. To achieve this, a recycle stream is needed to providerefrigeration and minimize ethylene losses to tail gas. In effect, theethylene loss to tail gas is controlled by the recycle separator drum330 temperature. A lower temperature will reduce the ethylene loss inthe tail gas 336, but will create a larger recycle, increasing the PGCpower consumption as well as the ethylene and propylene refrigerationcycle power consumption. For example, the simulation diagram shown inFIG. 8 obtains ethylene recovery of about 98.6 percent with a powerconsumption of 43,369 kw for PGC's and refrigeration; but if ethylenerecovery is pushed to 99.3 percent, then the PGC/refrigerationcompression power consumption increases by 6.7 percent. Therefore, thetradeoff is between additional ethylene product gain versus the cost ofthe additional power consumption, and economic optimization depends onthe ethylene value and power or fuel costs.

EXAMPLE 2

In this example, the embodiment of FIG. 7 was simulated as in Example 1on a commercial Aspen simulator using the simulation diagram shown inFIGS. 9A and 9B (“FIG. 9”) with selected pressures (oval balloons) andtemperatures (hexagonal balloons). The absorber 782 is used in place ofthe separator drum 330. The partially condensed stream from the cold box728 is sent to the absorber 782, which has just a few trays and nocondenser or reboiler, so the additional capital cost compared to theseparator drum 330 is minimal. The vapor stream 726 from the secondarydemethanizer feed separator (a −70.6° C./−95.1° F. drum) is furtherchilled through the cold box 728 down to just −100° C. (−148° F.),compared to −126° C. (−195° F.) in the Example 1 scenario. The partiallycondensed stream is then fed to the bottom tray in the absorber 782. Anethane liquid stream 784 from the C2 splitter bottoms 758 is chilled to−100° C. (−148° F.) through an exchanger 786 (see FIG. 7) in the coldbox 728. Mass transfer takes place in the absorber 782, wherein ethylenein the vapors in the partially condensed stream is absorbed in theliquid ethane. The overhead vapor (tail gas) stream 736 from theabsorber 782, which is passed through cold box 728 for recovery ofrefrigeration, contains much less ethylene than in the FIG. 3 embodimentof Example 1. The bottoms liquid from the absorber 782 is expandedacross valve 731, passed through cold box 728 for refrigerationrecovery, and recycled via line 732 to the suction of the first stagePGC 702.

The use of the ethylene absorber 782 can obtain much higher ethylenerecovery with a very small increase of power consumption. Compared toExample 1, for example, an ethylene recovery of 99.5 percent uses only2.1 percent more power. In addition, this embodiment chills the processgas to only 100° C. (−148° F.), well above the nitrated-gum-formationtemperature, thus enhancing the safety of the process

Certain embodiments and features have been described using a set ofnumerical upper limits and a set of numerical lower limits. It should beappreciated that ranges from any lower limit to any upper limit arecontemplated unless otherwise indicated. Certain lower limits, upperlimits and ranges appear in one or more claims below.

All numerical values are “about” or “approximately” the indicated value,and take into account experimental error and variations that would beexpected by a person having ordinary skill in the art.

Various terms have been defined above. To the extent a term used in aclaim is not defined above, it should be given the broadest definitionpersons in the pertinent art have given that term as reflected in atleast one printed publication or issued patent. Furthermore, allpatents, test procedures, and other documents cited in this applicationare fully incorporated by reference to the extent such disclosure is notinconsistent with this application and for all jurisdictions in whichsuch incorporation is permitted.

1) A process for recovering olefins from a feed stream, comprising:supplying the feed stream at a primary distillation pressure, includingif required compressing the feed stream in a primary compression stage;distilling the feed stream at the primary distillation pressure in aprimary distillation zone to obtain a primary overhead vapor streamenriched in ethylene and one or more ethylene-lean bottoms streams;chilling the primary overhead vapor stream at a pressure less than 30kg/cm2 (430 psia) in a first cooling stage to recover a first partialcondensate stream and a first-stage vapor effluent; refluxing theprimary distillation zone with at least a portion of the first partialcondensate stream; further chilling the first-stage vapor effluent torecover at least a second partial condensate stream and a second-stagevapor effluent; feeding the at least second partial condensate streamand any remaining portion of the first partial condensate stream to ademethanizer to recover a methane-rich overhead stream and a bottomsstream essentially free of methane and lighter components; fractionatingthe bottoms streams from the primary distillation zone and thedemethanizer into respective streams consisting essentially ofhydrocarbons selected from the group consisting of ethylene, ethane,propylene, propane, C4's, C5+and combinations thereof; further chillingthe second-stage vapor effluent in a cold section and phase-separatingthe resulting mixed vapor-liquid stream in one or more stages to obtainadditional condensate and a vapor tail gas stream essentially free ofethylene, wherein the additional condensate is vaporized to a relativelylower pressure to provide refrigeration for the condensation and to forma low pressure recycle vapor stream. 2) The process of claim 1 furthercomprising passing the feed stream in contact with a catalyst, adsorbentor combination thereof to remove at least one contaminant selected fromthe group consisting of acetylene, arsenic, mercury, carbonyl sulfide,nitrogen oxides, oxygen, and combinations thereof. 3) The process ofclaim 1 further comprising passing the primary overhead vapor stream incontact with a catalyst, adsorbent or combination thereof to remove atleast one contaminant selected from the group consisting of acetylene,arsenic, mercury, carbonyl sulfide, nitrogen oxides, oxygen, andcombinations thereof. 4) The process of claim 1 further comprisingcompressing the primary overhead vapor stream in a secondary compressionstage to a discharge pressure effective to provide reflux for theprimary distillation tower. 5) The process of claim 4 wherein theprimary distillation pressure is from 7 to 21 kg/cm2 (100 to 300 psia)and the discharge pressure from the secondary compression stage isgreater than the primary distillation pressure and less than 28 kg/cm2(400 psia). 6) The process of claim 5 wherein the discharge pressurefrom the secondary compression stage is from 3.5 to 7 kg/cm2 (50 to 100psia) greater than the primary distillation pressure. 7) The process ofclaim 5 wherein the primary distillation pressure is from 7 to 11 kg/cm2(100 to 160 psia) and the discharge pressure from the secondarycompression stage is from 10.5 to 17.5 kg/cm2 (150 to 250 psia). 8) Theprocess of claim 4, further comprising recycling the overhead streamfrom the demethanizer into the primary overhead vapor stream upstreamfrom the secondary compression stage. 9) The process of claim 8, whereinthe demethanizer consists essentially of an unrefluxed stripper column.10) The process of claim 1, wherein the primary distillation zonecomprises a depropanizer and the process further comprises fractionatingthe bottoms stream from the depropanizer in a debutanizer to obtainrespective streams consisting essentially of C4's and C5's and heavierhydrocarbons, and fractionating the bottoms stream from the demethanizerin a deethanizer, a C2 splitter and a C3 splitter to obtain respectivestreams consisting essentially of ethylene, ethane, propylene andpropane. 11) The process of claim 10, comprising exporting a portion ofan overhead stream from the deethanizer. 12) The process of claim 10,comprising exporting a portion of the first-stage vapor effluent. 13)The process of claim 10, wherein the deethanizer is refluxed with a sidedraw from the C2 splitter. 14) A process for recovering olefins from afeed stream, comprising: supplying the feed stream at a distillationpressure; distilling the feed stream at the distillation pressure in adistillation zone to obtain a overhead vapor stream enriched in ethyleneand one or more ethylene-lean bottoms streams; chilling the overheadvapor stream at a pressure less than 30 kg/cm2 (430 psia) in a firstcooling stage to recover a first partial condensate stream and afirst-stage vapor effluent; further chilling the first-stage vaporeffluent to recover at least a second partial condensate stream and asecond-stage vapor effluent; feeding the at least second partialcondensate stream and any remaining portion of the first partialcondensate stream to a demethanizer to recover a methane-rich overheadstream and a bottoms stream essentially free of methane and lightercomponents; fractionating the bottoms streams from the distillation zoneand the demethanizer into respective streams consisting essentially ofhydrocarbons selected from the group consisting of ethylene, ethane,propylene, propane, C4's, C5+and combinations thereof, further chillingthe second-stage vapor effluent in a cold section and phase-separatingthe resulting mixed vapor-liquid stream in one or more stages to obtainadditional condensate and a vapor tail gas stream essentially free ofethylene, wherein the additional condensate is vaporized to a relativelylower pressure to provide refrigeration for the condensation and to forma low pressure recycle vapor stream. 15) The process of claim 14,further comprising compressing the feed stream in a compression stage.16) The process of claim 14, further comprising refluxing thedistillation zone with at least a portion of the first partialcondensate stream. 17) The process of claim 14, further comprisingchilling the second-stage vapor effluent in a cold section andphase-separating the resulting mixed vapor-liquid stream in one or morestages to obtain additional condensate and a vapor tail gas streamessentially free of ethylene, wherein the additional condensate isvaporized to a relatively lower pressure to provide refrigeration forthe condensation and to form a low pressure recycle vapor stream.